The eerie howling of wolves, which sometimes can be heard over areas of up to 50 square miles, has long captivated the human imagination, spawning folklore in cultures around the planet. Now, scientists have found that wolves howl more when a ranking pack member or close companion leaves the group. The findings, published today (Aug. 22) in the journal Current Biology, indicate that social relationships, not physiological drives, explain why a wolf howls.

Co-author Friederike Range of the University of Veterinary Medicine in Vienna, Austria, told BBC News that when wolves howl, they are telling each other just how important they are. They howl differently depending on which wolf leaves the pack.

“We didn’t know there was some flexibility on how much they howl depending on their relationship. The amount of howling is really defined by the quality of the relationship,” Dr Range said.

In the study, researchers took away one wolf at a time from a captive wolf pack held inside a large enclosure at the Wolf Science Center at the University of Vienna. While the researchers kept each wolf away from its companions for a 45-minute period, they measured the howling rates of the animals left behind. They found that the howling rate was directly related to how much positive interactions–such as grooming and playing together–the remaining wolves had with the removed wolf.

Howling rates were also related to the removed wolf’s status within the pack, researchers said, with higher rates occurring when the more dominant member left. But even when the dominance factor was taken into account, the link between howling and the strength of social relationships remained strong, Range told BBC. Interestingly, the researchers observed that when a wolf was only removed to an spot close by their pack, its companions didn’t howl. A howling response was triggered only when the wolf was taken to an area further away.

The researchers also measured the levels of the stress hormone cortisol from saliva samples of the howling wolves. The tests demonstrated that stress levels were not strongly related to howling rates. These results run counter to a widely-held theory holding that animal vocalizations are produced as an automatic reaction to stress or emotional states.

According to Range, little is known about why wolves howl or what information they may be conveying. She said that future studies could investigate the meaning of wolf howls. More and more research has shown that the wolf, a highly intelligent creature, has strong family ties and complex social relationships. The new study’s findings shed important light on the question of how much animal vocalization can be thought of as voluntary, the researchers said.